Auto-zeroing current sensing element

Abstract

A current sensor for sensing alternating current in a current-carrying conductor includes a substantially C-shaped magnetic core having first and second ends with a gap therebetween. A current sensing element, which includes a microresonant element, is positioned proximate the gap between the first and second ends of the magnetic core such that the element oscillates when exposed to an alternating magnetic field. A biasing coil is attached to the micro-resonant element for carrying a bias current. The micro-resonant element resonates, or vibrates, at a standard oscillating frequency when no current is present in the biasing coil, and resonates at a modified oscillating frequency when current is present in both the biasing coil and the current-carrying conductor. The magnitude of current in the current-carrying conductor is determined as a multiple of the product of a ratio of the standard oscillating frequency and the modified oscillating frequency, and the bias current.

What is claimed is: 
     
       1. A current sensor for generating a signal representative of alternating current in a current-carrying conductor, said current sensor comprising: a magnetic core having first and second ends with a gap therebetween and shaped to form a conductor opening, said current-carrying conductor extending through said conductor opening; and   a current sensing element including a biasing coil for carrying a bias alternating current, said sensing element being positioned relative to said gap between said first and second ends of said magnetic core so that said sensing element moves when exposed to alternating magnetic fields of the current-carrying conductor and the biasing coil.   
     
     
       2. A current sensor in accordance with claim 1 wherein said current sensing element further comprises a microresonant element, said biasing coil being affixed to said microresonant element. 
     
     
       3. A current sensor in accordance with claim 2 wherein said microresonant element comprises a micro-cantilever element which resonates in response to external magnetic fields. 
     
     
       4. A current sensor in accordance with claim 2 wherein said microresonant element comprises a micro-bridge element which resonates in response to external magnetic fields. 
     
     
       5. A current sensor in accordance with claim 2 wherein said microresonant element comprises first and second portions, said first portion being more compliant to an alternating magnetic field than said second portion. 
     
     
       6. A current sensor in accordance with claim 2 wherein said microresonant element exhibits an oscillation frequency f 0  in absence of current in said biasing coil and a modified oscillation frequency f in the presence of current in said biasing coil and said current-carrying conductor, enabling determination of current magnitude in the current-carrying conductor as a multiple of the product of a frequency ratio of f/f 0  and the current in said biasing coil. 
     
     
       7. A current sensor in accordance with claim 1 wherein said sensing element comprises a pressure membrane and a plate separated by at least one spacer, said biasing coil being affixed to said membrane, and an optic fiber secured at a first end to said plate. 
     
     
       8. A current sensor in accordance with claim 7 wherein each of said membrane and said plate includes a substantially reflective surface, each of said reflective surfaces facing the other of said reflective surfaces. 
     
     
       9. A current sensor in accordance with claim 7 wherein said membrane includes first and second portions, said first portion being more compliant to an alternating magnetic field than said second portion. 
     
     
       10. A current sensor in accordance with claim 9 further comprising a light source and optical coupler, said light source and optical coupler each being secured to a second end of said optic fiber, said light source being configured to transmit light signals to said membrane, said optical coupler being configured to separate light signals returning from said membrane, whereby the returning signals return at a frequency f 0  in the absence of current in said biasing coil, and return at a frequency f in the presence of current in said biasing coil and said current-carrying conductor, enabling determination of current in the current-carrying conductor as a multiple of the product of a frequency ratio f/f 0  and the current in said biasing coil. 
     
     
       11. A method for measuring current in a current-carrying conductor using a current sensor including a magnetic core having first and second ends with a gap therebetween and shaped to form a conductor opening, said current-carrying conductor extending through said conductor opening, a current sensing element, and a bias alternating current source, the current sensing element including a biasing coil affixed thereto, the current sensing element being positioned proximate the gap so that the current sensing element moves in response to a magnetic field produced by the biasing coil, the biasing coil being electrically coupled to the bias alternating current source, said method comprising the steps of: determining current sensing element displacement in the presence of current in the biasing coil; and   determining current magnitude in the current-carrying conductor from the determined current sensing element displacement.   
     
     
       12. A method in accordance with claim 11 wherein the current sensing element further includes a microresonant element, the biasing coil being affixed to the microresonant element, said microresonant element being positioned proximate the gap such that the microresonant element resonates in response to a magnetic field produced by the current-carrying conductor, and wherein the steps of determining the current sensing element displacement comprises the steps of: determining a standard oscillating frequency f 0  of the microresonant element; and   determining a modified oscillating frequency f of the microresonant element when current is present in the biasing coil.   
     
     
       13. A method in accordance with claim 12 wherein the step of determining current magnitude in the current-carrying conductor comprises the step of generating a frequency ratio f 0 . 
     
     
       14. A method in accordance with claim 12 wherein the step of determining a modified oscillating frequency f comprises the step of establishing a current in the biasing coil so that the magnetic field of the biasing coil interacts with the magnetic field of the current-carrying conductor. 
     
     
       15. A method in accordance with claim 11 wherein the current sensing element further includes a microresonant element, the biasing coil being affixed to the microresonant element, and the microresonant element being positioned proximate the gap such that the microresonant element resonates in response to a magnetic field produced by the current-carrying conductor, and wherein the step of determining the current sensing element displacement comprises the steps of: determining a standard oscillating amplitude A 0  of the microresonant element; and   determining a modified oscillating amplitude A of the microresonant element when current is present in the biasing coil.   
     
     
       16. A method in accordance with claim 11 wherein the current sensing element includes a pressure membrane and a plate separated by at least one spacer, the biasing coil being affixed to said membrane, an optic fiber secured at a first end to said plate and secured at a second end to a light source and optical coupler, the light source transmitting light signals through the optic fiber to the pressure membrane, the optical coupler separating light signals returning from the pressure membrane, and wherein the step of determining the current sensing element displacement comprises the steps of: determining a standard frequency f 0  of light signals returning from the pressure membrane; and   determining a modified frequency f of light signals returning from the pressure membrane.   
     
     
       17. A method in accordance with claim 16 wherein the step of determining current magnitude in the current-carrying conductor comprises the step of generating a normalized frequency ratio of f/f 0 . 
     
     
       18. A method in accordance with claim 11 wherein the current sensing element includes a pressure membrane and a plate separated by at least one spacer, the biasing coil being affixed to said membrane, an optic fiber secured at a first end to said plate and secured at a second end to a light source and optical coupler, the light source transmitting light signals through the optic fiber to the pressure membrane, the optical coupler separating light signals returning from the pressure membrane, and wherein the step of determining the current sensing element displacement comprises the steps of: determining a standard oscillating amplitude A 0  of the pressure membrane; and   determining a modified oscillating amplitude A of the pressure membrane when current is present in the biasing coil.